ISA Calculator UK 2026

Use GlobalCalqulate’s free ISA Calculator (UK, 2026) to estimate how your savings could grow inside an Individual Savings Account (ISA). Model Cash ISA interest or Stocks and Shares ISA returns, monthly contributions, annual top-ups, and investment growth rate. See projected ISA value over time, total contributions vs growth, and tax-free compounding — ideal for long-term savings and wealth building in the UK.

Last revised 3 September 2026

By Team GlobalCalqulate · About our editorial standards

ISA Details

ISA Type

Current ISA Balance

How this estimate was calculated

The taxable-account comparison runs the same contributions at the same 6.00% gross return outside an ISA, taxing each year's growth as it arises: your £1,000 Personal Savings Allowance is applied every year, and the excess at 20%.
For a stocks and shares ISA this is the pessimistic end of the range. An unwrapped investor is taxed on dividends and on gains when they sell, not on unrealised growth each year, so the real advantage is usually smaller than shown — though it grows once dividends exceed the £500 dividend allowance or gains exceed the annual CGT exemption.
The comparison assumes this is your only savings interest. Interest from other accounts uses up the same Personal Savings Allowance and would increase the tax on a taxable account.
The annual ISA allowance is not enforced by this calculator — it only tracks what you tell it you've already used. Check your own contributions against the current HMRC allowance.
Fees are modelled as a constant drag on the growth rate, not deducted as a separate charge — a simplification of how platform and fund charges actually apply.

Financial Disclaimer: This result is an educational estimate produced by applying published rates and thresholds to the figures you entered. It is not a tax determination: your actual liability depends on circumstances this calculator does not collect. Confirm your position with the relevant tax authority or a qualified accountant before acting on it. Full disclaimer.

Sources & methodology

Formula:: Net growth rate after fees
Reference:: HM Revenue & Customs
Rates for:: 2026/27
Figures verified:: September 2026 — checked against the primary source above
Scheduled re-verification:: April 2027

Calculation type: Projection from your assumptions. Uses 2026/27 rates and thresholds. Figures are taken from HM Revenue & Customs — see sources below.

Reviewed by Team GlobalCalqulate — Verifies each figure against the issuing authority's published source · Checked 2 September 2026

Your entries stay on this device. This calculator runs entirely in your browser — the figures you type are not sent to us or to anyone else. The page itself uses cookies for analytics and advertising, described in the privacy policy.

Formula & methodology

Formula used

Net growth rate after fees

net rate = growth rate − annual fee rate

Fees are modelled as a constant drag on the growth rate, the standard simplified way to reflect an ongoing charges figure without a separate fee ledger.

Monthly compounding, contribution by contribution

balance(m) = balance(m−1) × (1 + net monthly rate) + contribution(m)

Your starting balance and each month's contribution (monthly amount plus any annual top-up spread evenly across the year) compound forward one month at a time. The contribution itself steps up once a year by your chosen annual increase, so a running total is used rather than the closed-form annuity formula, which assumes a constant payment throughout.

Projected value and growth

growth = projected value − total contributions (starting balance + everything paid in)

Separating the two makes clear how much of the projected pot is money you paid in and how much is investment return.

Inflation-adjusted value

real value = projected value ÷ (1 + inflation rate)^years

Shows what the projected pot would buy at today's prices.

Step-by-step calculation

1. Combine your monthly contribution and annual top-up
The annual top-up is spread evenly into a monthly-equivalent amount so both compound together.
2. Apply fees to the growth rate
Your entered annual fee percentage is subtracted from the growth rate before compounding.
3. Compound month by month
Stepping the contribution up once per completed year by your chosen annual increase.
4. Split out growth and discount for inflation
Giving the projected value, contributions, tax-free growth, fees given up to charges, and the inflation-adjusted figure.
5. Track your allowance
Remaining allowance is your entered annual allowance minus what you say you've already used — not independently verified.

Limitations

This is a projection from a fixed assumed rate, not a forecast. Real investment returns vary year to year and can be negative — a stocks and shares ISA can fall in value, and sequence of returns means two portfolios averaging the same rate can end up far apart.
The annual ISA allowance is not enforced. You can enter a contribution above the subscription limit and the projection will still run, so check your own contributions against the current allowance.
Fees are modelled as a constant drag on the growth rate rather than deducted as a pound-and-pence charge, which is a simplification of how platform and fund charges actually apply.
Cash ISA interest rates are variable in practice and are not fixed for the term, as this projection assumes.
Transfers between ISA types, withdrawals, flexible ISA rules and the Lifetime ISA bonus and withdrawal charge are not modelled.
This is an illustration, not financial advice or a guarantee of investment performance.

This page is educational. It projects a future value from assumptions you choose, and a projection is not a forecast: real returns vary year to year, and the rules, limits and rates used here can change before the period modelled ends. Treat the output as one scenario among many, not a prediction of what you will have.

Sources & references

Individual Savings Accounts (ISA): overview and allowance — HM Revenue & Customs
Lifetime ISA — HM Revenue & Customs
Individual Savings Accounts (ISA) Manual — HM Revenue & Customs
Tax on savings interest: how much is tax free — HM Revenue & Customs
Tax on dividends — HM Revenue & Customs
Income Tax rates and Personal Allowances — HM Revenue & Customs

Help & FAQs

Frequently Asked Questions

Clear answers to common questions to help you use this calculator confidently.

Does the calculator assume a fixed rate of return?

Tap to view the answer

Yes, it generally assumes a constant annual rate of return. In reality, investment returns may fluctuate. You can adjust the rate to explore different scenarios.

Are ISA contribution limits included in the calculations?

Tap to view the answer

The calculator does not automatically enforce the annual ISA allowance. Users should ensure their inputs align with current UK rules. Results are meant for planning.

What happens if I increase my monthly contribution?

Tap to view the answer

Increasing contributions generally raises the projected future value. You can adjust this input to explore different scenarios. The calculator updates results accordingly.

How does changing the rate of return affect results?

Tap to view the answer

Higher assumed returns increase projected balances, while lower returns reduce them. You can modify the rate to see how estimates change. Results are indicative.

What if I start investing later?

Tap to view the answer

Starting later may reduce the projected future value due to a shorter compounding period. You can change the start point to compare scenarios. Outcomes may vary.

Need more help? Contact support or email globalcalqulate@gmail.com

We typically reply within 24–48 hours.

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How to Use This Calculator

Enter your starting balance

Add any existing ISA balance you already have saved or invested.

Add contributions

Enter your monthly contribution and optional yearly top-up amount.

Choose expected growth rate

Set an estimated annual growth rate (interest for Cash ISA or expected return for Stocks & Shares ISA).

View projection

See your projected ISA value, total contributions, and tax-free growth over time.

Understanding Your Results

Projected ISA value
The estimated future value of your ISA after applying contributions and compound growth.
Total contributions
The total amount of money you personally add to your ISA over the period.
Total growth
The difference between projected ISA value and total contributions, showing tax-free gains.
Growth rate
The annual rate used to estimate how quickly your ISA grows. This can represent interest or investment return.
Time horizon
The number of years your money remains invested or saved in the ISA.

Understanding ISAs in the UK

What is an ISA?

An Individual Savings Account (ISA) is a UK tax-efficient wrapper that allows savings and investments to grow without income tax or capital gains tax on returns.

Cash ISA vs Stocks & Shares ISA

Cash ISAs earn interest and are lower risk. Stocks & Shares ISAs invest in markets and can offer higher long-term growth, but values can rise and fall.

How compounding works

Compounding means you earn returns not only on your contributions but also on previous growth. Over long periods, this can significantly increase your ISA value.

ISA allowance

The annual ISA allowance is £20,000 for 2026/27, a limit frozen since 2017/18. It applies across all your ISAs combined, not per account — staying within it keeps your savings tax-free.

Formula & methodology

Formula used

Net growth rate after fees

net rate = growth rate − annual fee rate

Fees are modelled as a constant drag on the growth rate, the standard simplified way to reflect an ongoing charges figure without a separate fee ledger.

Monthly compounding, contribution by contribution

balance(m) = balance(m−1) × (1 + net monthly rate) + contribution(m)

Projected value and growth

growth = projected value − total contributions (starting balance + everything paid in)

Separating the two makes clear how much of the projected pot is money you paid in and how much is investment return.

Inflation-adjusted value

real value = projected value ÷ (1 + inflation rate)^years

Shows what the projected pot would buy at today's prices.

Step-by-step calculation

1. Combine your monthly contribution and annual top-up
The annual top-up is spread evenly into a monthly-equivalent amount so both compound together.
2. Apply fees to the growth rate
Your entered annual fee percentage is subtracted from the growth rate before compounding.
3. Compound month by month
Stepping the contribution up once per completed year by your chosen annual increase.
4. Split out growth and discount for inflation
Giving the projected value, contributions, tax-free growth, fees given up to charges, and the inflation-adjusted figure.
5. Track your allowance
Remaining allowance is your entered annual allowance minus what you say you've already used — not independently verified.

Limitations

This is a projection from a fixed assumed rate, not a forecast. Real investment returns vary year to year and can be negative — a stocks and shares ISA can fall in value, and sequence of returns means two portfolios averaging the same rate can end up far apart.
The annual ISA allowance is not enforced. You can enter a contribution above the subscription limit and the projection will still run, so check your own contributions against the current allowance.
Fees are modelled as a constant drag on the growth rate rather than deducted as a pound-and-pence charge, which is a simplification of how platform and fund charges actually apply.
Cash ISA interest rates are variable in practice and are not fixed for the term, as this projection assumes.
Transfers between ISA types, withdrawals, flexible ISA rules and the Lifetime ISA bonus and withdrawal charge are not modelled.
This is an illustration, not financial advice or a guarantee of investment performance.

Calculator inputs

ISA Details

Contributions3 options

Growth Assumptions5 options

ISA Allowance2 options

Investment Horizon1 option

Your results

Sources & methodology

Formula & methodology

Formula used

Net growth rate after fees

Monthly compounding, contribution by contribution

Projected value and growth

Inflation-adjusted value

Step-by-step calculation

Limitations

Sources & references

Does the calculator assume a fixed rate of return?

Are ISA contribution limits included in the calculations?

What happens if I increase my monthly contribution?

How does changing the rate of return affect results?

What if I start investing later?

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More United Kingdom calculators

Calculator inputs

ISA Details

Contributions3 options

Growth Assumptions5 options

ISA Allowance2 options

Investment Horizon1 option

Your results

Sources & methodology

How to Use This Calculator

Example ISA Growth (UK)

Understanding Your Results

Key Tips

Understanding ISAs in the UK

What is an ISA?

Cash ISA vs Stocks & Shares ISA

How compounding works

ISA allowance

Formula & methodology

Formula used

Net growth rate after fees

Monthly compounding, contribution by contribution

Projected value and growth

Inflation-adjusted value

Step-by-step calculation

Limitations

Sources & references

How to Use This Calculator

Example ISA Growth (UK)

Understanding Your Results

Key Tips

Understanding ISAs in the UK

What is an ISA?

Cash ISA vs Stocks & Shares ISA

How compounding works

ISA allowance